A unit of measurement gives a number a shared physical meaning. Instead of saying that a point is six lengths right and two lengths up, we can specify centimeters, inches, or another unit. section 1.1
Units make precise placement possible in publishing. The point, written pt, equals 1/72 of an inch, so a page can be described with convenient whole numbers rather than awkward decimal inches. An A4 page is about 595pt wide and 842pt tall. section 1.1
Positioning a heading
Using the bottom-left corner as the origin (0, 0), a heading near (80, 630) lies about 80pt across and 630pt up the page. The same coordinate idea can describe letters, words, paragraphs, drawings, and photographs. section 1.1
A unit also exposes choices that are easy to overlook:
- The coordinate system uses x for across and y for up. section 1.1
- The origin need not be at the bottom left; another corner and direction could be chosen. section 1.1
- Small units matter because page elements and character shapes require careful positioning. section 1.1
